Devices based on wide bandgap III-V semiconductor materials such as gallium nitride (GaN) play a major role in our modern world. They play critical roles in essentially all of our electronic devices and are instrumental in almost all of the machines and apparatus we rely on every day. Examples of such semiconductor devices include light emitting devices such as light emitting diodes and laser diodes, electronic devices such as Schottky diodes, pn diodes, bipolar junction transistor, field effect transistors, metal-oxide-semiconductor field-effect transistor, insulated gate bipolar transistors, high electron mobility transistors, and heterojunction bipolar transistors to name a few, along with light absorbing devices such as solar cells. Forming such GaN devices of the highest performance often requires epitaxial structures with minimum defect density and the highest crystal quality and purity. To achieve the low defect density and high crystal quality it is most optimum to grow the epitaxial device epitaxial layers on a native GaN substrates to form a pseudomorphic epitaxial structure that is relatively free from strain related defects that occur when growing on foreign substrates.
Unfortunately, the synthesis of GaN single crystal substrates has been an extraordinarily difficult task. The highly successful Czochralski method for silicon crystal growth would have impractical process requirements comparable to conditions very deep within the Earth's mantle. Alternative approaches have been investigated for growing GaN bulk substrates, such as hydride vapor phase epitaxy (HVPE) and ammonothermal growth. Additionally it is still a great challenge to scale up bulk GaN growth to larger wafer sizes. GaN substrates are currently available in 2″ diameter at high volume and recent announcements have revealed availability in 4″ in the near future, which is still drastically smaller than more mature substrate technologies such as 12″ single crystal silicon. At the current GaN wafer diameter and prices, the native substrate option is not economically feasible for realizing semiconductor devices in many applications, specifically light emitting diode applications and power electronic applications. Given the obstacles in GaN native substrate manufacturing, there has been substantial effort devoted to the epitaxy on foreign substrate materials. Common choices for GaN heteroepitaxy include sapphire, silicon carbide, and silicon. In the past decade, SiC and sapphire substrates have been widely used in nitride LEDs and RF transistors.
Light emitting diodes (LED) based on gallium nitride are lighting the world around us. An LED is a two-lead semiconductor light source. It is a basic pn-junction diode, which emits electromagnetic radiation when activated. The emission from an LED is spontaneous and is typically in a Lambertian pattern. When a suitable voltage is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor [1].
Appearing as practical electronic components in 1962 the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were also of low intensity, and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness [1].
The earliest blue and violet GaN-based LEDs were fabricated using a metal-insulator-semiconductor structure due to a lack of p-type GaN. The first p-n junction GaN LED was demonstrated by Amano et al. using the LEEBI treatment to obtain p-type GaN in 1989 [2]. They obtained the current-voltage (I-V) curve and electroluminescence of the LEDs, but did not record the output power or the efficiency of the LEDs. Nakamura et al. demonstrated the p-n junction GaN LED using the low-temperature GaN buffer and the LEEBI treatment in 1991 with an output power of 42 uW at 20 mA. The first p-GaN/n-InGaN/n-GaN DH blue LEDs were demonstrated by Nakamura et al. in 1993 [3]. The LED showed a strong band-edge emission of InGaN in a blue wavelength regime with an emission wavelength of 440 nm under a forward biased condition. The output power and the EQE were 125 uW and 0.22%, respectively, at a forward current of 20 mA. In 1994, Nakamura et al. demonstrated commercially available blue LEDs with an output power of 1.5 mW, an EQE of 2.7%, and the emission wavelength of 450 nm [4]. On Oct. 7, 2014, the Nobel Prize in Physics was awarded to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources” or, less formally, LED lamps [1].
LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, and camera flashes. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology.
A laser diode is a two-lead semiconductor light source that that emits electromagnetic radiation that is comprised primarily of stimulated emission. The laser diode is comprised of a gain medium that functions to provide emission through the recombination of electron-hole pairs and a cavity region that functions as a resonator for the emission of the gain medium. When a suitable voltage is applied to the leads to sufficiently pump the gain medium, the cavity losses are overcome by the gain and the laser diode reaches the so-called threshold condition, wherein a steep increase in the light output versus current input characteristic is observed. Unlike LEDs, laser diodes emit very directional light and have orders of magnitude higher spatial brightness. Moreover, above threshold, they do not suffer from the droop phenomenon that plagues LEDs.
Early visible laser technology comprised lamp pumped infrared solid state lasers with the output wavelength converted to the visible using specialty crystals with nonlinear optical properties. For example, a green lamp pumped solid state laser had 3 stages: electricity powers lamp, lamp excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The resulting green and blue lasers were called “lamped pumped solid state lasers with second harmonic generation” (LPSS with SHG) had wall plug efficiency of ˜1%, and were more efficient than Ar-ion gas lasers, but were still too inefficient, large, expensive, fragile for broad deployment outside of specialty scientific and medical applications. To improve the efficiency of these visible lasers, high power diode (or semiconductor) lasers were utilized. These “diode pumped solid state lasers with SHG” (DPSS with SHG) had 3 stages: electricity powers 808 nm diode laser, 808 nm excites gain crystal, which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. As high power laser diodes evolved and new specialty SHG crystals were developed, it became possible to directly convert the output of the infrared diode laser to produce blue and green laser light output. These “directly doubled diode lasers” or SHG diode lasers had 2 stages: electricity powers 1064 nm semiconductor laser, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm green light. These lasers designs are meant to improve the efficiency, cost and size compared to DPSS-SHG lasers, but the specialty diodes and crystals required make this challenging today.
Based on essentially all the pioneering work on GaN LEDs described above, visible laser diodes based on GaN technology have emerged. Currently the only viable direct blue and green laser diode structures are fabricated from the wurtzite AlGaInN material system. The manufacturing of light emitting diodes from GaN related materials is dominated by the heteroepitaxial growth of GaN on foreign substrates such as Si, SiC and sapphire. Laser diode devices operate at such high current densities that the crystalline defects associated with heteroepitaxial growth are not acceptable. Because of this, very low defect-density, free-standing GaN substrates have become the substrate of choice for GaN laser diode manufacturing. Unfortunately, such bulk GaN substrates are costly and not widely available in large diameters. For example, 2″ diameter is the most common bulk GaN c-plane substrate size today with recent progress enabling 4″ diameter, which are still relatively small compared to the 6″ and greater diameters that are commercially available for mature substrate technologies.
Semiconductor power electronic devices are a key class of semiconductor devices that hugely affect the world we live in. Power electronics started with the development of the mercury arc rectifier. Invented by Peter Cooper Hewitt in 1902, it was used to convert alternating current (AC) into direct current (DC). From the 1920's on, research continued on applying thyratrons and grid-controlled mercury arc valves to power transmission. Uno Lamm developed a valve with grading electrodes making mercury valves usable for high voltage direct current transmission. In 1933 selenium rectifiers were invented [5].
In 1947 the bipolar point-contact transistor was invented by Walter H. Brattain and John Bardeen under the direction of William Shockley at Bell Labs. In 1948 Shockley's invention of the bipolar junction transistor improved the stability and performance of transistors, and reduced costs. By the 1950's, semiconductor power diodes became available and started replacing vacuum tubes. In 1956 the Silicon Controlled Rectifier (SCR) was introduced by General Electric, greatly increasing the range of power electronics applications [6]. In the 1960's the switching speed of bipolar junction transistors allowed for high frequency DC/DC converters. In 1976 power MOSFETs became commercially available. In 1982 the Insulated Gate Bipolar Transistor (IGBT) was introduced [6].
Power electronic devices may be used as switches, or as amplifiers. An ideal switch is either open or closed and so dissipates no power; it withstands an applied voltage and passes no current, or passes any amount of current with no voltage drop. Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line, and the power dissipation inside the device is large compared with the power delivered to the load [5].
The very high breakdown voltages, high electron mobility and saturation velocity of GaN has made it an ideal candidate for high-power and high-temperature power electronic devices, as evidenced by its high Johnson's Figure of Merit. Potential markets for high-power/high-frequency devices based on GaN include microwave radio-frequency power amplifiers (such as used in high-speed wireless data transmission) and high-voltage switching devices for power grids. A potential mass-market application for GaN-based RF transistors is as the microwave source for microwave ovens, replacing the magnetrons currently used. The large band gap means that the performance of GaN transistors is maintained up to higher temperatures than silicon transistors. The first gallium nitride metal semiconductor field-effect transistors (GaN MESFET) were experimentally demonstrated in 1993 and they are being actively developed. Other devices include pn junction diodes, Schottky diodes, field effect transistors (FET), junction field effect transistor (JFET), insulated gate bipolar transistors (IGBT), heterojunction bipolar transistors (HBT), and high electron mobility transistors (HEMT). Such devices can be deployed in many applications such as in automobiles, hybrid electric automobiles, cell phones, iphones, ipads, computers, and others [1g 7].
In 2010 the first enhancement mode gallium nitride transistors became generally available. These devices were designed to replace power MOSFETs in applications where switching speed or power conversion efficiency is critical. These transistors, also called eGaN FETs, are built by growing a thin layer of GaN on top of a standard silicon wafer. This allows the eGaN FETs to maintain costs similar to silicon power MOSFETs, but with the superior electrical performance GaN [7].